I doubt EDI would of survived if EZY had ATRs, with less seats the cost per seat would of been significantly higher and with 8 flights a day from STN with Ryanair and easyJet with seats often being less than £10 there would be no demand from SEN.
Also the Air Berlin loads to MAN from STN were not great, I just don't think the route is viable. SEN is difficult to get to for anyone who doesn't live in the catchment area. By the time you get to SEN, pay for parking/or train, the flight time then getting from MAN airport to the city. The cost and time would not make sense.
BA from MAN only works because of the connections at LHR, same as Leeds Bradford. LGW was discontinued don't forget.